This invention relates to a device for measuring an internal pressure in a living body or organism by inserting a catheter into the living body.
Such an internal pressure measuring method has heretofore been extensively used for clinical or other purposes, for example, in an invasive blood pressure monitoring system.
In most of the conventional devices, a pressure transducer (pressure-electric signal transducer) is disposed externally of the living body, and the internal pressure in the living body is fed to the transducer via a catheter introduced into the living body, thereby effecting the measurement. In this method, an atmosphere-opening point of the pressure measuring transducer is set at the same level or height as a pressure reference point (for example, the heart (at the point of tricuspid valve) when measuring the blood pressure) of the object to be measured, and the pressure-receiving surface of the transducer is caused to be open to the atmosphere to thereby effect a zero point adjustment, and thereafter the pressure measurement is carried out. However, this method has a vital drawback. More specifically, when the position of the object to be measured with respect to the transducer is varied, the hydrostatic pressure (plus or minus) of the liquid in the catheter is added, and this causes an error in measurement value. Particularly when measuring a pressure usually of several mmHg such as the central venous pressure, a position variation of 1 cm causes an error (variation) of 0.73 mmHg. This error can not be ignored. For this reason, from a clinical point of view, it has been difficult to effect a significant measurement without re-adjusting the position of the transducer, and it has been still more difficult to effect the monitoring for a long period of time in a stable manner.
In conventional devices of another type under consideration, a transducer is mounted on the distal end of a catheter. Such a device obviates the need for the re- adjustment of the position of the transducer necessitated by the position variation; however, the zero point adjustment of the transducer can be made only before the catheter is introduced into the living body, and once the catheter is introduced into the living body, the zero point adjustment can not be made in the body. Thus, such conventional device is also not suited for a reliable, long-term monitoring.